The present invention relates to a combined structure comprising parts of different coefficients of thermal expansion which may produce an adverse effect on the structure and, more particularly, to such type of structure that comprises a combination of an optical part (e.g., an optical mirror) of an optical device using a movable galvanic mirror and its support. The combined structure according to the present invention is of such nature that in case a structure part such as an optical mirror of which a comparatively high degree of flatness is required is combined with its support, no particular trouble takes place.
FIGS. 1 through 3 show a conventional combined structure comprising a plurality of parts having different coefficients of thermal expansion, which parts are combined with each other through connecting members.
The prior art technique shown in the above figures is that an optical part 3, such as a mirror, is placed on a reference surface 2 of a support 1 made of e.g., aluminum and connecting members 6, 7, 8 and 9 are applied to the corners formed between the side wall surfaces 4 and 5 of the optical part 3 and the reference surface 2 of the support 1 so that the part 3 is force-connected to the reference surface 2.
The prior art technique has had such disadvantages that when a stress F.sub.3 is generated in the direction of the width of the optical part 3 in a low temperature atmosphere due to the difference in coefficient of thermal expansion between the support 1 and the optical part 3 as shown in FIG. 2, a bending moment M.sub.3 is applied on the optical part 3, and when a stress F.sub.4 is generated in the longitudinal direction of the part 3 as shown in FIG. 3, a bending moment is applied to the part 3. As a result, the optical part 3 deforms to become defocussed due to a torsional strain on the part 3.
A phenomenon of defocussing of the conventional optical part is shown in FIG. 4, which is a graph illustrating the relationship between the temperature and defocussing of the part 3.
As will be clear from FIG. 4, the amount of defocussing of the optical part 3 due to a tortional strain increases from around 15.degree. C.
Further, other prior art techniques relating to a structure comprising a combination of parts having different coefficients of thermal expansion are disclosed in the Japanese Laid-open Patent Publication No. 52-50704 and in Japanese Laid-open Utility Model Publications Nos. 56-119147 and 57-33434.
However, countermeasures against the strain applied to a combined structure due to the difference in coefficient of thermal expansion between the parts forming the structure have not been taken so far.